Learning outcomes in almost 15% of English and Maths primary classes are unsatisfactory, according to Department of Education inspectors.
The inspectors also said that teacher preparation in almost a quarter of these classes is not up to a proper standard.
Publishing a report on this issue this afternoon, the department described the findings as surprising and worrying.
The study was based on unannounced inspections of more than 800 English classes and 500 Maths lessons.
